Extended investigation of the test specimen thickness (TST) effect on the fracture toughness (Jc) of a material in the ductile-to-brittle transition temperature region as a difference in the crack tip constraint − What is the loss of constraint in the T

To answer this question, several well-known constraint parameters were investigated to determine their correlations with the TST effect and the bounded nature of Jc for increasing TST. Based on the elastic-plastic finite element analysis results, it was demonstrated that the well-known stress triaxiality factor Θ = (hydrostatic stress)/(von Mises stress), measured at 4δtc (crack-tip opening displacement at fracture load Pc), exhibited a good correlation with the decreasing and subsequently bounded nature of Jc for increasing TST. Θ started to decrease at some load level before fracture for relatively thin specimens, and this was the loss of constraint in the TST effects on Jc.